Friction stir welding (FSW) is a welding process in which a cylindrical, shouldered welding tool having a pin protruding from the shouldered surface is rotated and plunged into a joint line between two portions of a fabrication preform. The portions of the fabrication preform are typically of metal, but the process may also be used on various plastics and polymers. Typically the pieces are clamped together to prevent relative motion between the pieces during the welding process. Frictional heat is generated between the pin and the material of the portions of the fabrication preform. This heat causes the adjacent material to soften (generally without reaching its melting point). The tool is then moved relative to the portions of the fabrication preform along a weld line while the shoulder of the tool is pressed against the surface of each portion of the fabrication preform. The softened material is transferred from the leading edge of the pin to the trailing edge of the pin. The shoulder helps form a flat weld that is even with the surface of each portion of the fabrication preform and that, after cooling, bonds the portions of the fabrication preform together.
The power required to spin a friction stir tool increases significantly when the process is applied to thicker and/or harder materials. Consequently, current friction stir welding processes are generally best suited for welding aluminum or other comparatively soft metals with thicknesses of less than approximately one inch (2.5 cm). What are needed therefore are improved friction stir welding processes that may be applied to thicker and/or harder materials.